In optical devices and electron devices such as a solar cell, a photodiode, a flat panel display (FPD) and a light-emitting diode (LED), in order to maximally extract their characteristics, a technique for diffusing light is an essential technique, and in particular, a technique for improving light scattering ability at all wavelength regions is essential. As such an approach for scattering light, there are mainly two approaches, which are, for example, as described in Patent Literature 1:
(1) an approach for forming fine asperities (hereinafter, textures) on a surface to increase surface roughness, thereby causing optical scattering; and
(2) an approach for causing scattering by a flat surface and a built-in optical scattering element.
In the case of imparting light scattering ability to a transparent conductive film, the approach (2) of the above two methods results in deterioration in electrical characteristics, in particular, increase in electrical resistance, and thus this approach is a very complicated method, when taking into consideration a process for preparing a transparent conductive film. Therefore, the approach (1) is usually used.
It has been hitherto examined to allow the surface of a transparent conductive film to have various shapes using this approach in order to improve light scattering ability. For example, in Patent Literature 2, a surface roughness has been increased by adjusting film formation conditions to accomplish a high light scattering ability. In order to exceed this, in Patent Literature 3, light scattering ability has been improved by providing fine mountain-like relieves on the surface of a transparent conductive film and forming a plurality of finer asperities on each mountain.
Recently, surface textures suitable for improving the light scattering ability of a transparent conductive film have been found, and for example, it has been reported in Non Patent Literature 1 that concave lens-like textures of a single size are formed on the surface of a thin film to thereby obtain more favorable light scattering ability than the mountain-like textures. In this method, the concave lens-like textures are formed by subjecting a transparent conductive film once formed to etching by an acid.
On the other hand, with respect to such concave lens-like textures, it has been reported in Patent Literature 1 that a plurality of lenses of several μm are prepared on the inner curved surface of a lens of several tens of μm to make it possible to further improve light scattering ability. However, in this Literature, since the textures are formed by molding on a polymer surface using a mold, it is difficult to perform microfabrication below several μm, and a base material on which the textures can be formed has been limited to polymers. Therefore, a method for more finely and complicatedly forming the concave lens-like textures on the surface of a substrate is required in order to impart a higher light scattering ability to a transparent conductive film.
Furthermore, even in the case of forming the concave lens-like textures of a single size on a transparent conductive film, it is necessary to regulate the temperature of a base plate to 300° C. or higher in order to ensure the transparency of the transparent conductive film, and thus the base plate on which the transparent conductive film is formed is limited to materials (glass, metal plate, ceramics and the like) that can withstand high temperatures. If it is possible to reduce such a film formation temperature, it enables film formation on a base plate using an organic substance such as a polymer, thereby enabling the selectivity of material of a base plate to be considerably extended.